The present invention relates to the preparation of wood laminates, and more particularly to the preparation of wood laminates from high moisture content wood plies.
In preparing wood laminates such as plywood from high moisture plies in the past, it was invariably found that a large number of defects in lamination would occur. It is believed that this problem is directly related to the formation of steam between the plies and a blow out of the stream when pressure is released upon completion of the molding cycle. Thus, it has been necessary in the past to incur sizable operating and capital costs to assure consistently low moisture contents for plies employed in making wood laminates.
In preparing wood laminates such as plywood, it is conventional to provide an odd number of plies. The laminates are typically formed by applying an adhesive to the mating surfaces of the plies, pressing them at essentially ambient temperature for a period of time effective to consolidate the plies into a unitary panel, and then subjecting the consolidated panel to a final curing pressing between hot platens under elevated temperature and pressure for a period of time sufficient to finally set the thermosetting adhesive. When working with wood plies other than extremely dry ones, experience has shown that an excessively large number of panels must be rejected because of poor lamination.
It is our present understanding that the hot platens vaporize the moisture within the plies and drive it toward the center. The temperature gradient throughout the laminate drives the moisture toward the center ply from both ends of the hot platens. In this manner, substantially all the moisture is driven to the center core. At the same time, the resin is curing from the hot platens toward the center glue line. As the temperature builds in the center plies, so does the vapor pressure, and as the press is opened, the built up vapor or steam seeks an avenue of escape and blows the panel.
This problem increases in severity with the moisture content of the plies. In published tests in the Forest Products Journal, Volume 23, No. 10, Chen and Rice indicate that veneer moisture content variation had a dramatic effect on assembly time relationships, and that prolonged assembly times generally produced the highest wood failure results at higher veneer moisture content levels. Adverse effects were obtained even though the moisture contents employed in the study were only within the range of from about 1.3 to about 8.0%. To bring the freshly prepared wet plies down to this moisture content range, substantial drying is necessary. Thus, even with substantial drying, the prior art experienced undue problems of laminate failures.
It would be advantageous to have a process which reduced the need for drying high moisture content plies and veneers and enabled the use of plies and veneers with relatively high moisture contents. This is especially true in view of the recent economic and regulatory stimulus to conserve energy. Reduced drying requirements would also result in savings in capital costs and processing time. Additionally, it would be advantageous to have a process capable of handling high moisture content plies and veneers with reduced failure rates because such a process would reduce the criticality of moisture control and thus make the process more flexible.